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Introduction to Damper Tank Stabilizers
A ship is subjected to rolling and pitching not only during the time it is moving in the sea, but also when it is standing stationary at one position. Stabilizers in ships are mainly of two types – fin stabilizers and tank stabilizers. Fin stabilizers are used to balance and keep a ship safe during rough weather from excessive rolling and pitching whereas tank stabilizers are used when the ship is not moving, for e.g. at anchorage.
The working of ship's fins stabilizers depends on the speed of the ship. This means that if the ship is not moving the stabilizers cannot be used to oppose the rolling motion of the ship, generated by the force of the waves. However, in tank stabilizers it’s totally a different game. Tank stabilizers are used to ensure the equilibrium of the ship when it is stationary and without any motion.
As the name suggests, tank stabilizers are made up of tanks which are located in the extreme transverse region of the ship. Tank stabilizers provide a righting or an anti-rolling motion when the ship is not moving. The anti-rolling force takes place as a result of delayed flow of fluid in a suitably positioned transverse tank. The working of this type of ship stabilizer as a ship stability apparatus is independent of the ship's speed.
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Construction and Arrangement of Tank Stabilizers in Ships
The system of operation followed for tank stabilization is known as passive operation. Ships having this method for stabilization have a wing tank system located at greater heights and on each side. The greater heights of the tanks permits a larger water build up in order to resist the roll. However, the water is filled into the tanks in such a way that the rolling action doesn't fill the tanks up to the brim. This is done with the help of air ducts, which contains valves operated by a roll sensing device. The air ducts are attached between the two wing tanks and the air pressure is regulated to control the fluid flow inside the tanks. Take a look at the picture to have a clear idea.
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Operation of Damper Tank Stabilizers
To understand the operation of tank stabilizers, consider a mass of water in an athwartship tank of a ship. The tank is filled with water up to a specified level (Predetermined). When the ship is stationary there is no movement in the water inside the tank. However, as the ship rolls the water inside the tank gains momentum after a certain period of time. This means that when the ship is finishing its roll and is about to turn, the movement of water inside the tank, which initiates at a later stage, opposes the rolling motion of the ship from inside and thus balances the ship. The similar process is repeated when the ship rolls on the other side. In this way stabilization of the ships takes place with the help of tank stabilizers. The athwartships tank is also known as flume. The main reason this operation is known as a passive operation, is that it uses water flow which is activated by gravitational force.
However, accommodating tank stabilizers into a ship's design might turn out dangerous to its own stability. For this reason, the tanks should be carefully and specifically designed for a particular ship using data from the model tests. The level of water inside the tanks should also be adjusted according to the ship's loading condition and limit. Efforts should also be made to reduce the free surface effect that results from the moving water, which eventually disturbs the stability of the ship.
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Biran, A. (2003) Ship Hydrostatics and Stability. Oxford: Butterworth-Heinemann Publishing